Nickel-chromium-cobalt-molybdenum alloy

ABSTRACT

A nickel-chromium-cobalt-molybdenum alloy includes (in weight %) Cr 21-23%, Fe 0.05-1.5%, C 0.05-0.08%, Mn≦0.5%, Si≦0.25%, Co 11-13%, Cu≦0.15%, Mo 8.0-10.0%, Ti 0.3-0.5%, Al 0.8-1.3%, P&lt;0.012%, S&lt;0.008%, B&gt;0.002-&lt;0.006%, Nb&gt;0-1%, N≦0.015%, Mg≦0.025%, Ca≦0.01%, V 0.005-0.6%, optionally W in contents between 0.02-max. 2%, Ni rest as well as smelting-related impurities, in the form of tubes, sheets, wire, bars, strips or forgings, wherein the alloy satisfies the following formula: 
       X3=5−50, wherein
 
               X                 3     =     100   *       X                 1       X                 2               and 
       X1=C+5N 
       and 
       X2=0.5Ti+Nb+0.5V.

The invention relates to a nickel-chromium-cobalt-molybdenum alloy with excellent strengths and creep characteristics as well as extraordinary resistance to high-temperature corrosion.

EP 2039789 A1 discloses a nickel-base alloy for a turbine motor for a steam engine, containing: C 0.01 to 0.15%, Cr 18 to 28%, Co 10 to 15%, Mo 8 to 12%, Al 1.5 to 2%, Ti 0.1 to 0.6%, B 0.001 to 0.006%, Ta 0.01 to 0.7%, rest nickel and unavoidable impurities. This composition is supposed to have an elevated mechanical strength with simultaneous retention of forging characteristics.

A nickel-chromium-molybdenum-cobalt alloy with a special carbide morphology that imparts a better creep rupture strength at elevated temperatures to the alloy has become known through EP 0358211 B1 or EP 2204462 A1. The alloy consists (in % by wt) of 15 to 30% chromium, 6 to 12% molybdenum, 5 to 20% cobalt, 0.5 to 3% aluminum, up to 5% titanium, 0.04 to 0.15% carbon, up to 0.02% boron, up to 0.5% zirconium, up to 5% tungsten, up to 2.5% niobium or tantalum, up to 5% iron, up to 0.2% rare earth metals, up to 0.1% nitrogen, up to 1% copper, up to 0.015% sulfur, up to 0.03% phosphorus and up to 0.2% magnesium or calcium, rest nickel except for impurities.

Even though the alloys may contain up to 2.5% Nb or Ta, these elements impair the resistance to cyclic oxidation, which occurs particularly strongly with simultaneous presence of chromium and aluminum.

Such alloys have been used in practice for many decades and are known under the designation “alloy 617”. It has been found that structural parts made from such alloys have a certain tendency to stress cracks in the temperature range from 550 to 850° C. This has been evident in particular at welded joints of thick-walled components. Internal stresses in conjunction with carbide precipitates are regarded as causes for this. To some extent it has been possible to eliminate this by a multi-hour heat treatment at ca. 1,000° C., but in some cases it has been possible to perform such a heat treatment not at all or only with great difficulties.

It is the task of the invention to so improve this known and also proven alloy by purposeful modification of individual alloying elements that the indicated disadvantages are no longer present.

This task is accomplished by a nickel-chromium-cobalt-molybdenum alloy consisting of (in % by wt)

Cr 20-24% Fe ≦1.5% C 0.03-0.1% Mn ≦0.5% Si ≦0.25% Co 10-15% Cu ≦0.15% Mo 8.0-10.0% Ti 0.1-0.8% Al 0.3-2.0% P <0.012% S <0.008% B >0.002 and <0.008% Nb >0-2% N ≦0.015% Mg ≦0.05% Ca ≦0.01% V ≦1.0%

Ni Rest as well as smelting-related impurities.

A preferred alloy composition is represented as follows (in % by wt):

Cr 21-23% Fe 0.05-1.5% C 0.05-0.08% Mn ≦0.5% Si ≦0.25% Co 11-13% Cu ≦0.15% Mo 8.0-10.0% Ti 0.3-0.5% Al 0.8-1.3% P <0.012% S <0.008% B >0.002 and 0.006% Nb >0-1.8% N ≦0.015% Mg ≦0.025% Ca ≦0.01% V ≦0.6%

Ni Rest as well as smelting-related impurities.

It is of particular advantage when the contents of B and Nb are adjusted as follows:

-   -   B 0.002-0.005%     -   Nb>0-1.0%.

The Mn content is advantageously ≦0.3%. If necessary, the alloy may contain W as a further element in contents between 0.02 and 2%.

It is of further advantage when the vanadium content in the alloy according to the invention is adjusted to >0 to ≦0.6%.

Surprisingly, it has been found that the precipitation of chromium carbide stringers can be suppressed by purposeful alloying with Nb and/or V as well as B. Thereby the tendency toward formation of stress cracks during welding is considerably reduced during operation.

According to a further idea of the invention, the alloy according to the invention satisfies the following formula:

X3=5−50, wherein

${X\; 3} = {100*\frac{X\; 1}{X\; 2}}$ and

X1=C+5N

and

X2=0.5Ti+Nb+0.5V.

If necessary for the increase of the ductility and for the elimination of stresses, the alloy according to the invention may be subjected to a heat treatment in the temperature range between 800 and 1,000° C., preferably at 980° C. In this way the proportion of carbides should advantageously be >0.9%. By purposeful adjustment especially of the contents of Nb, V and B, such a heat treatment may now be performed without difficulties.

By virtue of the subject matter of the invention, a highly creep-resistant alloy for operating temperatures between 500 and 1,200° C. is obtained.

The alloy according to the invention is usable not only in the form of tubes, sheets, wire, bars, forgings or castings and strips, but also for welded constructions. Preferred areas of application are gas turbines, the construction of furnaces and power plants, the petrochemical industry and the field of nuclear power engineering.

In Table 1, an alloy that may be regarded as belonging to the prior art is compared with 5 variants V1 to V5 according to the invention.

TABLE 1 VdTÜV Prior Material art V2 Sheet Typical V1 Nb 0.5 V3 V4 V5 485 analysis Nb 0.5 V 0.2 V 0.2 V 0.65 Mo high Element % by wt % by wt % by wt % by wt % by wt % by wt % by wt Ni Rest Rest Rest Rest Rest Rest Rest Cr 20.0- 22.08 22 22 22 21.9 21.5 23.0 Co 10.0- 11.54 12.2 12.2 12.4 12.4 12.4 13.0 Mo 8.0- 8.65 8.4 8.4 8.4 8.4 9.5 10.0 Ti 0.20- 0.39 0.41 0.4 0.4 0.4 0.41 0.50 Al 0.60- 1.09 0.86 0.84 0.84 0.82 0.88 1.50 Fe max. 2.0 1.22 0.32 0.36 0.1 0.23 0.03 Mn max. 0.1 0.02 0.02 0.02 0.02 0.02 0.70 Si max. 0.2 <0.01 <0.01 <0.01 <0.01 0.01 0.70 C 0.050- 0.062 0.05 0.05 0.05 0.05 0.065 0.100 P max. 0.003 <0.001 <0.01 0.002 0.002 0.002 0.012 S max. <0.002 <0.001 <0.001 <0.001 <0.001 <0.001 0.008 As max. 0.001 <0.01 <0.01 <0.01 <0.01 <0.01 0.010 B max. 0.001 0.0033 0.0034 0.0034 0.0033 0.0028 0.001 Pb max. 0.0002 <0.005 <0.005 <0.005 <0.005 <0.005 0.007 V 0.02 <0.01 0.18 0.18 0.6 <0.01 N 0.011 <0.01 <0.01 <0.01 <0.01 <0.01 Nb 0.02 0.55 0.5 <0.01 <0.01 <0.01 W 0.4 0.1 0.1 0.1 0.1 0.1

In Table 2, alloys that may be regarded as belonging to the prior art and five variants V1 to V5 according to the invention are compared with regard to the dissolution behavior of the carbides.

TABLE 2 Solution annealing temp. M6C primary Solvus Nb V Mo carbide Cr carbide Variant % by wt % by wt % by wt ° C. ° C. Prior art 0 0 8-10 1250-1290 990-1000 V1 0.55 <0.01 8.4 1237 1096 V2 0.5 0.18 8.4 1207 1153 V3 <0.01 0.18 8.4 1228 1133 V4 <0.01 0.6 8.4 1214 1182 V5 <0.01 <0.01 9.5 1290  839

In Table 3, an alloy that may be regarded as belonging to the prior art and 5 variants V1 to V5 according to the invention are compared with regard to the ductility (SSRI test at 700° C.)

TABLE 3 Reduction of Elongation area (Z) (A) Variant Comment ° C. ° C. Prior Without 7.5 5 art boron V1 14 8.5 V2 11 8.5 V3 21 24 V4 42 21 V5 20 10 

1-17. (canceled)
 18. Nickel-chromium-cobalt-molybdenum alloy, consisting of (in % by wt) Cr 21-23% Fe 0.05-1.5% C 0.05-0.08% Mn ≦0.5% Si ≦0.25% Co 11-13% Cu ≦0.15% Mo 8.0-10.0% Ti 0.3-0.5% Al 0.8-1.3% P <0.012% S <0.008% B >0.002-<0.006% Nb >0-1% N ≦0.015% Mg ≦0.025% Ca ≦0.01% V ≦0.005-≦0.6%, optionally W in contents between 0.02-max. 2% Ni Rest as well as smelting-related impurities, in the form of tubes, sheets, wire, bars, strips or forgings, wherein the alloy satisfies the following formula: X3=5−50, wherein ${X\; 3} = {100*\frac{X\; 1}{X\; 2}}$ and X1=C+5N and X2=0.5Ti +Nb+0.5V.
 19. Alloy according to claim 18, with (in % by wt): B>0.002-<0.005%.
 20. Alloy according to claim 18, with (in % by wt) Mn≦0.3%.
 21. Alloy according to claim 18, wherein the proportion of carbides is >0.9%.
 22. Use of the alloy according to claim 18 as forgings for components in gas and steam turbines.
 23. Use of the alloy according to claim 18 as welded constructions for gas and steam turbines.
 24. Use of the alloy according to claim 18 as boiler structural parts for power plants in power engineering.
 25. Use of the alloy according to claim 18 in construction of furnaces and power plants.
 26. Use of the alloy according to claim 18 in the petrochemical industry as well as in the area of nuclear power engineering.
 27. Use of the alloy according to claim 18 as castings for gas and steam turbines as well as in the construction of furnaces and power plants.
 28. Use according to claim 27 as centrifugally cast structural parts. 